Aluminium-based grain refiner

ABSTRACT

ALUMINIUM-BASED GRAIN REFINER, which contains zinc, titanium and carbon, synthesized by means of aluminium fusion and the subsequent addition of titanium, zinc and carbon. The microstructure of the master alloy is comprised by an alpha-aluminium matrix, intermetallic phase of TiAl 3  and fine particles of a ternary compound of titanium-zinc-carbide. The titanium content is at least 1% and at the most 20%, the carbon content is at least 0.01% (100 ppm) and at the most 3% and the zinc content is equal to or over 1%.

The invention refers to an aluminium-based grain refiner or refining agent.

More specifically, the purpose of the invention focuses on a new aluminium alloy (also called “master alloy”) that contains aluminium, zinc, titanium and carbon, which is obtained or synthesized by means of aluminium fusion and the subsequent addition of titanium, zinc and carbon. After obtaining the desired composition and its homogenization, the melted alloy is moulded by casting into the desired shape. The microstructure of the master alloy is comprised of a matrix of alpha aluminium, intermetallic phase of TiAl₃ and fine particles (particle size within the interval between nm and a few μm) of a ternary compound of titanium-zinc-carbon. The practical application of this quaternary master alloy is to refine the grain of the aluminium and its alloys.

FIELD OF APPLICATION

The field of application of this invention is found in the aluminium metallurgical industry, specifically for refining aluminium grain and its alloys.

BACKGROUND OF THE INVENTION

As it is known, aluminium grain refining and that of its alloys is a widely used industrial practice. The refiner of standard commercial grain is the master alloy of Al—Ti—B, which contains particles of TiB₂ and which is available commercially with different contents of Ti and of B. The most commonly used master alloy is the Al—5% Ti—1% B in the form of rods (percentage compositions in weight unless otherwise indicated).

It is known that the master alloys of Al—Ti—B are effective for general applications although they cause the following defects in certain specific applications, due to the presence of large particles and agglomerations of TiB₂:

pores in the aluminium foil,

streaks and surface defects in sheet metal and similar products that require a good surface finish,

contraction cracks and cracking in ingots, especially in some high-resistance alloys, such as 7010/7050, that are used in the aeronautics industry.

Furthermore, larger quantities of Al—Ti—B must be added for the refining of alloy grain for high temperature that contain Cr and Zr, which are believed to be a poison for the conventional grain refining of Al—Ti—B.

In view of the existence of the abovementioned problems in a wide field of specific applications, the principal inventor, Dr. Abinash Banerji, developed a boron-free ternary grain Al—Ti—C refiner that contains particles of TiC, which form nuclei of aluminium grains (Dr. Abinash Banerji, Doctoral Thesis at the Technical University of Berlin, 1987).

The production procedure of this ternary grain refiner was later patented in 1985 (U.S. Pat Nos. 4,748,001 and 4,842,821, EU patents no. 0214220 (United Kingdom), 0214220 (France), 0214220 and P3679263.2 (Germany), 0214220 (Netherlands), Australian patent no. 595187, Canadian patent no. 1289748, Japanese patent no. 2121452, Norwegian patent no. 167589).

For more than a decade and up to now, the Al—Ti—C grain refiner has been produced commercially worldwide by various manufacturers, although the Al—Ti—C master alloys are available with different contents of Ti and C. The commonly used master alloy, however, has an approximate composition of Al—3% Ti—0.2% C.

Despite being an industrial practice for more than a decade, the Al—Ti—C grain refiner available up to now has not been capable of solving effectively the problems associated with the conventional Al—Ti—B grain refiner, since greater quantities are needed to be added to obtain a grain size level similar to that obtained with the commercial Al—Ti—B grain refiner.

In view of the above, the purpose of this invention is focused on developing another refiner that exceeds the performance of the Al—Ti—C grain refiners commercially available up to now, and that efficiently solves the well-known problems associated with the conventional Al—Ti—B grain refiner, having to point out that the applicant does not know of any alloy or any aluminium-based master alloy that has been produced that contains zinc, titanium and carbon and that shows the capacity of refining the grain, such as that which is claimed in this invention.

EXPLANATION OF THE INVENTION

Specifically, the grain refiner that is subject of the invention is produced as follows:

In a furnace, commercially-available pure aluminium (generally 99.7% aluminium) is melted at the normal aluminium fusion temperatures (approximately 700-1000° C.), the necessary quantity of titanium is added in the form of titanium sponge or of titanium scrap (for example, chips). Another source of titanium can be a salt that contains titanium, for example, K₂TiF₆. In this last case, initially a binary mixture of Al—Ti can be produced, by means of adding the required quantity of the salt to the melted aluminium, and after the binary Al—Ti mixture is ready, and the slag is removed from the melted surface, the subsequent process can commence. Alternatively, a master alloy of Al—Ti can also be melted directly.

Next, pure zinc or an alloy that contains zinc (for example, a Zn—Al alloy) is added to the melted mass followed by carbon powder which can be graphite or amorphous carbon.

The usual process is for the addition of the titanium to be followed by zinc and carbon, although the sequence of the addition of the elements is not a limitation for the invention. The addition can be of the three elements together or of one after the other.

The holding time needed for the alloy in the furnace after all the elements have been added is usually between 30 and 60 minutes so that the complete reaction of the three elements produces the ternary carbide particles of titanium, zinc and carbon. The holding time will depend on the composition of the casing, on its volume, on the fusion temperature and the type of furnace. In the case of fusion by induction, a stirring effect can reduce the holding time.

The master alloy obtained is cast with the usual procedure in “waffles”, blocks or bars that can be subsequently processed in the form of rods or wire. In accordance with the state of the art, continuous casting equipment can be used directly along with a rolling device in order to process directly the master alloy in rolls of bars or wire. Alternatively, other casting procedures can be used and shaped to obtain the grain refiner in the desired shape and size.

The master alloy is apt for use in grain refining of aluminium and of its alloys. It can be used in the form of ingots, bars or rods according to the grain refining techniques.

It is important to highlight that the microstructure of the master alloy obtained by means of the described procedure contains the three following phases:

Matrix phase, comprised of α—Al.

Primary intermetallic phase of TiAl₃ aluminide.

Ternary phase of titanium-zinc-carbide.

Furthermore, some traces of free carbon can be found in the microstructure.

In summary, in the standard alloy the titanium content is at least 1% and at the most 20%, the carbon content is at least 0.01% (100 ppm) and at the most 3% and the zinc content is at least 1%.

In an example of preparing the grain refiner that is subject of this invention, in FIGS. 1 and 2 the standard microstructure is presented of an Al—6% Zn—3.5% Ti—0.4% C alloy obtained in form of a rod.

Tests were also carried out of refining TP1 grain using the new refiner and the results were compared with those obtained with the commercially available grain refiners of Al—Ti—B and Al—Ti—C. FIGS. 3, 4, 5 and 6 show the typical macrographs of the conducted TP1 tests.

It can be observed that in the conventional 2-minute short test, conducted with commercial 99.7%-pure aluminium, with the new Al—6% Zn—3.5% Ti—0.4% C refiner, a grain size of 110 μm was obtained, which is slightly finer than that obtained with the Al—5% Ti—1% B (115 μm) and considerably finer than that obtained with Al—3.5% Ti—0.2% C (170 μm).

DESCRIPTION OF THE DRAWINGS

For a better interpretation of the invention, this descriptive report is accompanied by some photographs in which is illustrated, but not limited to the typical microstructure and the results obtained with a grain refiner with a composition of Al—6% Zn—3.5% Ti—0.4% C, such as that which is the subject of this invention, according to the principles of the claims.

In these photographs:

FIG. 1 shows the typical microstructure of an Al—6% Zn—3.5% Ti—0.4% C grain refiner obtained in the form of a rod.

FIG. 2 shows the same typical microstructure of an Al—6% Zn—3.5% Ti—0.4% C grain refiner obtained in the form of a rod, at a lesser number of magnifications.

FIG. 3 shows the typical macrograph of Al 99.7% without adding the refiner.

FIG. 4 shows the typical macrograph of Al 99.7% refined with 2 Kg/MT of Al—5% Ti—1% B in a rod.

FIG. 5 shows the typical macrograph of Al 99.7% refined with 2 Kg/MT of Al—3.5% Ti—0.2% C in a rod.

FIG. 6 shows the typical macrograph of Al 99.7% refined with 2 Kg/MT of Al—6% Zn—3.5% Ti—0.4% C in a rod. 

1. ALUMINIUM-BASED GRAIN REFINER, also called master alloy, of the type used for refining the grain of aluminium and of its alloys, characterised by the fact that it contains zinc, titanium and carbon; by its being produced or synthesized by means of aluminium fusion and the subsequent addition of titanium, zinc and carbon; and by the microstructure of the master alloy being comprised by an alpha-aluminium matrix, intermetallic phase of TiAl₃ and fine particles (particle size within the interval between nm and a few μm) of a ternary compound of titanium-zinc-carbide.
 2. ALUMINIUM-BASED GRAIN REFINER, according to claim 1, characterized by the fact that the titanium content is at least 1% and at the most 20%, the carbon content is at least 0.05% (100 ppm) and at the most 3% and the zinc content is at least 1%.
 3. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the titanium content is 1%.
 4. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the titanium content is greater than 1% but less than 20%.
 5. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the titanium content is 20%.
 6. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the carbon content is 100 ppm.
 7. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the carbon content is greater than 100 ppm but less than 3%.
 8. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the carbon content is 3%.
 9. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the zinc content is 1%.
 10. ALUMINIUM-BASED GRAIN REFINER, according to claim 2, characterized by the fact that the zinc content is greater than 1%. 